1. Field of the Invention
This invention relates to the handling of glass sheets and, in particular, to a horizontal press bending arrangement wherein heat softened glass sheets are transferred into a shaping station and deposited between a pair of vertically aligned, full surface shaping molds without contacting the upper major surface of the heat softened glass sheet.
2a. Technical Considerations
Shaped glass sheets are widely used as windows in vehicles such as automobiles and the like. The glass sheets are generally heat strengthened, i.e., annealed or tempered depending upon the desired use of the window in the vehicle. To be suitable for such applications, flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the framing defining the window opening in the vehicle. It is also important that the windows meet stringent optical requirements so that the windows are free of optical defects that would tend to interfere with the clear viewing through the viewing area.
The commercial production of shaped glass sheets for such purposes commonly includes heating flat glass sheets to their heat softening temperature, shaping the heated glass to a desired curvature, and cooling the shaped glass in a controlled manner to a temperature below the strain point temperature of the glass, e.g., tempering to strengthen the glass and increase its resistance to damage resulting from impact. During such an operation, a glass sheet is generally conveyed along a substantially horizontal path that extends through a tunnel-type furnace. The glass sheet is one of a series of sheets and is heated to its heat softening temperature. The glass sheet is then transferred, for example, by a vacuum pick-up and shuttle arrangement, to a shaping station adjacent the furnace where the glass sheet is by press bending, sag bending, or other well known glass shaping After shaping, a transfer and tempering ring conveys the shaped glass sheet into a cooling station for rapid cooling.
Other shaping operations employ the use of a stationary transfer platen that supports the heat softened glass sheets from above without contacting the glass sheet and a shuttling shaping ring arrangement. The lower facing surface of the platen includes two sets of holes: one set of holes pulls a vacuum along the facing surface to draw the glass towards the platen and a second set of holes provides pressurized gas along the facing surface to prevent the glass from contacting the platen. An open center shaping ring is positioned below the platen to receive the glass. After vacuum is terminated, the glass sheet drops onto the ring to shape the glass by gravity. The ring with the glass sheet supported thereon, moves from beneath the platen into a cooling station.
An advantage in using a non-glass surface contacting transfer arrangement is that it reduces the incidents of scratching and/or marring during transfer from the furnace to the heating station. In addition, in instances where the glass sheet includes a ceramic border or some other type of added surface pattern on its upwardly facing major surface, the glass sheet can be transferred to the shaping station without contacting the coated major surface and possibly rubbing or scuffing the pattern such that the glass part is defective or optically unacceptable.
As automotive stylists strive to provide more aerodynamic vehicle designs, the bent shapes of the glass sheets are becoming increasingly more complex. These shapes that include configurations that cannot be achieved by using conventional sag bending techniques, such as reverse curvatures and small radius bends, require tight dimensional and contour tolerances. In order to repeatedly and effectively produce these complex configurations, full surface bending molds have been used.
It would be advantageous to provide a glass sheet handling and shaping arrangement wherein the glass sheets have minimal surface contact with the heating and conveying operation and the sheets can be shaped to complex configurations.
2b. Patents of Interest
U.S. Pat. No. 3,223,443 to Misson teaches a glass sheet handling apparatus for supporting glass sheets heated to their heat deformation temperature without distorting or marring the glass sheet surface. The sheet is supported from above by a holder that establishes a pressure and exhaust zone between the upper surface of the sheet and the lower surface of the holder. A net pressure differential is established above the sheet which is less than the ambient pressure by an equal amount to the weight of the glass sheet. As a result, the glass sheet is supported by, but floats below, the lower surface of the holder.
U.S. Pat. Nos. 4,578,103 and 4,615,724 to Fackelman teach the combination of a glass sheet heating conveyer, a top-side transfer apparatus, similar to that taught in Misson, to support the heat softened glass sheet, and a curved outline mold positioned below the holder to receive the heat softened glass sheet from the transfer apparatus for forming. The transfer apparatus includes holes to draw a vacuum at the face of the apparatus and holes to provide pressurized gas along the face of the apparatus. The holes may be inclined so as to help move the supported glass sheet along the lower surface of the apparatus. The glass sheet is dropped onto an open center shaping ring, wherein it deforms by gravity to conform to the peripheral configuration of the ring. In U.S. Pat. No. 4,578,103, the glass sheet is conveyed off of the conveyor rolls and directly beneath the transfer apparatus. The transfer ring is moved below the top side transfer apparatus to receive the heat softened glass sheet for sag bending. In U.S. Pat. No. 4,615,724, the transfer apparatus engages the glass sheet while the glass is supported on the heating conveyor and moves vertically upward to allow the shaping ring to be positioned under the supported glass sheet.
U.S. Pat. Nos. 4,282,026; 4,361,432; 4,437,871; and 4,437,872 to McMaster et al. each teach a drop forming apparatus wherein a hot glass sheet is engaged within a heating furnace by a stationary upper vacuum pick-up positioned above the furnace conveying rolls and subsequently is deposited on a shuttling carrier mold ring. The downwardly facing surface of the vacuum pick-up can be planar or curved. The pick-up can reciprocate vertically to engage the glass, or auxiliary lifters can be positioned between the furnace rolls and beneath the hot glass sheet to lift the glass for engagement with the vacuum pick-up. The ring moves beneath the vacuum supported glass sheet and the vacuum is terminated to drop the hot glass sheet on the ring and effect shaping. The ring subsequently shuttles from its pick-up transfer station to a quench unit that rapidly cools the shaped glass. Throughout the operation, the vacuum pick-up remains horizontally stationary within the furnace and the glass is transferred directly to a movable ring mold.
U.S. Pat. Nos. 4,227,908; 4,229,199; 4,229,200; 4,233,049; and 4,280,828 to Seymour teach shaping glass sheets by drop forming. A heat softened glass sheet exits a furnace and is positioned on a support bed below a stationary flat vacuum pick-up. The pick-up lifts the glass sheet and moves upward to allow a contoured shaping ring to be positioned under the sheet. Vacuum is disengaged and the sheet drops on the ring. The force generated by the impact of the glass sheet on the ring provides the bending force required to shape the sheet and conform it to the desired configuration.
U.S. Pat. No. 4,297,118 to Kellar et al. teaches a shuttling deformable vacuum mold that engages a heated glass sheet within a heating furnace. While still in the furnace, the mold deposits the shaped glass sheet on a shuttling tempering ring that is positioned beneath the mold. After depositing the glass, the vacuum mold shuttles to a position outside of the furnace to cool prior to reentering the furnace to engage the next glass sheet. The tempering ring transfers the glass sheet from the furnace to a quenching station to temper the glass.
U.S. Pat. No. 4,517,001 to McMaster teaches the use of a traveling vacuum holder with a downwardly facing engaging surface to lift a heated glass sheet and transfer the sheet onto a carrier ring mold within the heating furnace wherein the heated glass sheet is bent under the force of gravity on the mold. The bent glass sheet is subsequently removed from the furnace to a quench unit to temper the bent glass.